<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <button>点我</button>
  <script>
    const btn = document.querySelector("button")
    // btn.onclick = function () {
    //   alert(123)
    // }

    // btn.onclick = () => {
    //   alert(123)
    // }

    // btn.onclick = () => {
    //   alert(123)
    //   return
    // }

    // btn.onclick = () => {
    //   return alert(123)
    // }

    btn.onclick = () => alert(123)
  </script>
</body>

</html>